After doing an extensive search of the web, I found that Ambion offers
proteinase K dissolved in 50 mM Tris pH 8, 3 mM CaCl2, 50%
glycerol. To make a stock from powder, dissolve the proteinase K
at 20 mg/ml in 100 mM Tris pH 8, 6 mM CaCl2 and then dilute
with an equal volume of 100% glycerol. Store the Proteinase K at
-20.

It turns out that proteinase K activity is stimulated by the presence
of CaCl2; hence, CaCl2 is in the storage buffer.